1. Field of the Invention
The present invention relates to a display apparatus which uses a display panel having a matrix form. More particularly this invention relates to a display apparatus having a driving circuit which drives a data line of the display panel.
2. Description of the Related Art
A matrix liquid crystal display, especially an active matrix liquid crystal display, is generally constructed as shown in FIG. 1, where a matrix display panel 1 comprises a plurality of liquid crystal display elements arranged in rows and columns at intersections of data lines 2 extending vertically, i.e., in the Y direction, and address lines 3 extending horizontally, i.e., in the X direction. The liquid crystal display is shown to be equipped only with capacitors 4 and switching devices 5. Capacitors 4 retain the activating voltage applied to the liquid crystal. Switching devices 5 control the supply of the activating voltage to capacitors 4. In practice, matrix display panel 1 further includes display electrodes corresponding to pixels, a common transparent electrode corresponding to the display electrodes, and a liquid crystal layer sandwiched between each display electrode and the common transparent electrode. The activating voltage is applied to the display electrodes.
An X-driver circuit 6 drives data lines 2 according to image signals. A Y-driver circuit 7 drives address lines 3 according to scanning signals. In particular, X-driver circuit 6 receives an image signal corresponding to one horizontal scanning line and activates plural data lines 2 simultaneously. Y-driver circuit 7 activates address lines 3 successively each time data lines 2 are activated. Thus, the horizontal scanning lines of display panel 1 are driven successively.
Where the image signal applied to the display device takes a digital form, X-driver circuit 6 is required to convert its input digital image signal into analog form which is used to drive data lines 2. The prior art X-driver circuit having a digital-to-analog converter function comprises N stages of shift registers, N latch circuits for retaining an input digital image signal corresponding to one horizontal scanning line, and N D/A converters which receive the output signals from the N latch circuits and convert the signals into analog form.
In the X-driver circuit of this structure, the number of D/A converters must be identical to the number of pixels N contained in the horizontal scanning line. Therefore, if the number of pixels N contained in one horizontal scanning line for an input digital signal is large, or if the number of bits per pixel is large, then the X-driver circuit must be made very large. If this driver circuit is fabricated as an integrated circuit (IC), the area of the chip increases.
Where the input image signal takes an analog form, the X-driver circuit includes N sample-and-hold circuits which are connected to the input image signal in parallel and are sequentially enabled so that each sample-and-:old circuit holds the input image signal corresponding to one pixel in each horizontal scanning line. The image signals are then delivered simultaneously to data lines 2. In this case, when the number of pixels N contained in one horizontal scanning line is large, the sampling interval of the sample-and-hold circuits must be made short so as to sample-and-hold only image signals coming from corresponding pixels.
In order to reduce the sampling period of the sample-and-hold circuits, it is necessary either to increase the width of the gate of the sampling transistor (normally a MOS transistor) to reduce the resistance or to reduce the capacitance of each holding capacitor. As a result, the offset voltage of the sample-and-hold circuits increases. That is, the sampling period and the offset voltage of the sample-and-hold circuits have a conflicting relation to each other. Therefore, if the sampling interval is shortened, the offset voltage increases, thus deteriorating the image quality.